Electricity: power supply or regulation systems – In shunt with source or load – Using choke and switch across source
Reexamination Certificate
2002-12-05
2004-03-23
Berhane, Adolf (Department: 2838)
Electricity: power supply or regulation systems
In shunt with source or load
Using choke and switch across source
C323S282000
Reexamination Certificate
active
06710582
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a DC-DC converter and an electronic device including such a DC-DC converter. More particularly, the present invention relates to a DC-DC converter in which switching loss is reduced and to an electronic device including such a DC-DC converter.
2. Description of the Related Art
FIG. 6
is a circuit diagram of a step-down DC-DC converter. In
FIG. 6
, the DC-DC converter
1
includes a DC power supply Vin, a rectifier diode D
1
, a choke coil L
1
, a MOSFET Q
1
functioning as a switching element, a smoothing capacitor C
1
, a diode D
2
, a capacitor C
2
, a capacitor C
3
, and a control circuit
2
. The diode D
2
is a body diode of the MOSFET Q
1
, and the capacitor C
2
is a drain-source junction capacitance, that is, a parallel capacitance, of the MOSFET Q
1
. The capacitor C
3
is an anode-cathode junction capacitance, that is, a parallel capacitance, of the rectifier diode D
1
.
The cathode of the rectifier diode D
1
is connected to one end of the choke coil L
1
and the anode thereof is grounded. The source of the MOSFET Q
1
is connected to the node between the rectifier diode D
1
and the choke coil L
1
and the drain thereof is connected to one end of the DC power supply Vin. The other end of the DC power supply Vin is grounded. The other end of the choke coil L
1
is connected to an output terminal Po. The smoothing capacitor C
1
is connected between the output terminal Po and the ground. The control circuit
2
is connected between the output terminal Po and the gate, which is the control terminal, of the MOSFET Q
1
.
Now, the operation of the DC-DC converter
1
will be described. The control circuit
2
ON/OFF-drives the MOSFET Q
1
. First, when the MOSFET Q
1
is ON, a current flows to the choke coil L
1
through the MOSFET Q
1
by an input voltage supplied from the DC power supply Vin. When the MOSFET Q
1
is turned OFF, a current flows to the choke coil L
1
through the rectifier diode D
1
due to the excitation inertia of the choke coil L
1
. By repeating this operation, a voltage according to the duty of the ON/OFF operation of the MOSFET Q
1
is output from the output terminal Po. The control circuit
2
changes the duty of switching of the MOSFET Q
1
according to an output voltage in order to perform PWM control so that the output voltage is kept constant regardless of variations in the input voltage and a load.
FIG. 7
is a circuit diagram of a step-up DC-DC converter. In
FIG. 7
, elements which are the same as those in
FIG. 6
are denoted by the same reference numerals. In
FIG. 7
, the DC-DC converter
5
includes a DC power supply Vin, a rectifier diode D
3
, a choke coil L
2
, a MOSFET Q
2
functioning as a switching element, a smoothing capacitor C
1
a diode D
4
, a capacitor C
4
, a capacitor C
5
, and a control circuit
2
. The diode D
4
is a body diode of the MOSFET Q
2
, and the capacitor C
4
is a drain-source junction capacitance, that is, a parallel capacitance, of the MOSFET Q
2
. The capacitor C
5
is an anode-cathode junction capacitance, that is, a parallel capacitance, of the rectifier diode D
3
.
The anode of the rectifier diode D
3
is connected to one end of the choke coil L
2
and the cathode thereof is connected to the output terminal Po. The drain of the MOSFET Q
2
is connected to the node between the rectifier diode D
3
and the choke coil L
2
and the source thereof is grounded. The other end of the choke coil L
2
is connected to one end of the DC power supply Vin. The other end of the DC power supply Vin is grounded. The smoothing capacitor C
1
is connected between the output terminal Po and the ground. The control circuit
2
is connected between the output terminal Po and the gate, which is the control terminal, of the MOSFET Q
2
.
Now, the operation of the DC-DC converter
5
will be described. The control circuit
2
ON/OFF-drives the MOSFET Q
2
. First, when the MOSFET Q
2
is ON, a current flows to the choke coil L
2
and then to the MOSFET Q
2
by the input voltage from the DC power supply Vin so that the choke coil L
2
is excited. When the MOSFET Q
2
is OFF, a current flows from the DC power supply Vin through the choke coil L
2
and the rectifier diode D
3
. At this time, the voltage at one end of the choke coil L
2
is higher than that at the other end thereof because of its inertia. Therefore, when the voltage at the other end of the choke coil L
2
reaches the input voltage Vin, the voltage at the one end surpasses the input voltage Vin, and thus a step-up operation is realized. Then, a voltage that is increased by repeating this operation is output from the output terminal Po. As in the DC-DC converter
1
, the control circuit
2
changes the duty of switching of the MOSFET Q
2
according to an output voltage in order to perform PWM control so that the output voltage is kept constant regardless of variations in the input voltage and a load.
When the switching element of the DC-DC converter is ON, a current is applied to the switching element but an ON-resistance is almost zero and thus, almost no loss is caused. On the other hand, when the switching element is OFF, a voltage is applied to the switching element but a current is not applied thereto, and thus, almost no loss is caused.
However, in the DC-DC converters
1
and
5
, when the MOSFET Q
1
or Q
2
functioning as a switching element is turned ON/OFF, a voltage and a current are applied to the switching element for a moment, and large switching loss is caused at that time. Further, the current flowing through the MOSFET Q
1
or Q
2
and the rectifier diode D
1
or D
3
abruptly changes and thus, a high noise may be generated. Also, when the MOSFET Q
1
or Q
2
is turned ON, a surge recovery current flows from the cathode to the anode during a reversed recovery time of the rectifier diode D
1
or D
3
, which leads to great loss.
In order to overcome this problem, Japanese Patent No. 3055121 discloses a configuration for realizing zero-voltage switching and zero-current switching of a switching element by using resonance.
In this configuration, switching loss and noise can be reduced. However, a capacitance that is large enough to supply a load current is required as a resonance capacitor. Accordingly, a resonance period depending on a resonance capacitor and a resonance reactor is necessary at the time when the switching element (switching element 2) is turned ON/OFF. Thus, PWM control, in which ON-period and OFF-period of the switching element is further shortened, is not performed. As a result, a wide-range variation in the input voltage and output voltage are not adequately dealt with. Further, a sine-wave resonance current is added to the output current flowing through the switching element. Therefore, a switching element having a large current capacitance is required, which leads to an increase in the size and cost of the DC-DC converter.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a DC-DC converter in which switching loss and noise are greatly reduced, a wide-range variation in an input voltage and output voltage is dealt with, and an increase in the size and cost is prevented, and also provide an electronic device including such a novel DC-DC converter.
According to a preferred embodiment of the present invention, a DC-DC converter includes a rectifier diode, a choke coil, one end thereof being connected to one end of the rectifier diode, a first switching element, one end thereof being connected to the node between the rectifier diode and the choke coil through a resonance coil, a first diode connected in parallel to the first switching element, a second switching element, a series circuit including a capacitor and the second switching element and connected in parallel to a series circuit including the resonance coil and the rectifier diode, and a second diode connected in parallel to the second switching element. Each of the first and second switching elements and the re
Berhane Adolf
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
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